Photography of forming a direct positive image without employing a reversal step or using a negative film is well known.
A conventionally known process of forming a positive image with a known direct positive silver halide photographic material may be essentially be classified into the following two types, with certain exceptions, considering the practical usefulness of the process.
One type produces a direct positive image, where a previously fogged silver halide emulsion is used and the fogged nuclei (latent image) in the exposed area is broken by solarization or the Herschel effect for development to thereby obtain the intended direct positive image.
The other type produces a direct positive image, where a non-fogged internal latent image-type silver halide emulsion is used and the emulsion is, after imagewise exposure, subjected to surface development during or after a fogging treatment to thereby obtain the intended direct positive image.
The above-mentioned internal latent image-type silver halide photographic emulsion means a silver halide photographic emulsion of a type such that the silver halide grains therein have light-sensitive nuclei essentially in the inside thereof and a latent image is formed essentially in the inside of the grains by exposure.
The method of the latter type generally has a higher sensitivity than that of the former type and is therefore suitable for uses which require a high sensitivity. The present invention relates to the latter type.
Various techniques are known in this technical field. For instance, U.S. Pat. Nos. 2,592,250, 2,466,957, 2,496,875, 2,588,982, 3,317,322, 3,761,266, 3,761,276 and 3,796,577 and British Patents 1,151,363, 1,150,553 and 1,011,062 illustrates essential techniques known in the field.
Using known methods, direct positive photographic materials having a relatively high sensitivity can be obtained.
The details of the mechanism of forming direct positive images are given, for example, in T. H. James, The Theory of the Photographic Process, Ed. 4, Chap. 7, pages 182 to 193 and U.S. Pat. No. 3,761,276.
More specifically, it is considered that fogged nuclei are selectively formed on the surfaces of non-exposed silver halide grains because of the surface-desensitization action caused by a so-called internal latent image formed in the inside of the silver halide grains by the first imagewise exposure, and thereafter the thus fogged nuclei-containing emulsion is then subjected to a general so-called surface development to ultimately form a photographic image (direct positive image) in the non-exposed area.
Suitable means of selectively forming fogged nuclei, in general, include a so-called "light-fogging method" in which the complete surface of the light-sensitive layer (for example, British Patent 1,151,363) is subjected to a second exposure and a so-called "chemical fogging method" in which a nucleating agent is used The latter method is described in detail, for example, in Research Disclosure, Vol. 151, No. 15162 (issued in November, 1976), pages 76 to 78.
For forming a direct positive color image, an internal latent image-type silver halide photographic material is subjected to surface color development after or during fogging, and thereafter it is bleached and fixed (or bleach-fixed). After the bleach-fixing step, the material is generally rinsed in water and/or stabilized.
Investigations have been made to apply a direct positive color photographic material to a print-related fields because the characteristics involve forming a positive color image simply and rapidly.
However, a printing ink is quite different from a coloring dye used in a general color photographic material from the standpoint of the spectral characteristics.
For instance, a printing magenta ink is quite different from a magenta-coloring dye used in a color photographic material, as shown in the Figure. Accordingly, where the direct positive color photographic material of the present invention is applied to a print-related field, it is necessary for the color characteristics of the magenta-coloring dye in the color photographic material to be similar to those of a printing magenta ink.
The spectral absorption of a printing ink is extremely sharp in the long wavelength range and the short wavelength range has a large amount of blue absorption. In the case of a color photographic material, it is considered extremely difficult to form a colored dye having a similar spectral absorption characteristic as a printing ink has, from a coupler of one kind in the material.
Accordingly, incorporation of both a magenta-coloring coupler capable of forming a color dye having a sharp spectral absorption and an yellow-coloring coupler into the green-sensitive emulsion of the photographic material as a mixture of the two might be considered.
As a magenta-coloring coupler, a pyrazoloazole magenta coupler forming a color dye having a small amount of side-absorption of a cyan component and having a sharp spectral absorption is preferred.
However, based on present research, it has been found that incorporation of both an yellow coupler and a pyrazoloazole magenta coupler into the same light-sensitive emulsion layer often gives color dyes of different hues in the low-density portion and the high-density portion.
Additionally, where a direct positive color photographic material containing the above-mentioned yellow coupler and pyrazoloazole coupler in the same emulsion layer is stored at room temperature for a long period of time, a severe problem occurs in that the quality of the image formed is noticeably deteriorated. Precisely, in such a case, the maximum image density (D.sub.max) of the image formed is reduced and the minimum image density (D.sub.min) thereof is increased.
In the situation, development of a direct positive color photographic material which may be applied to a print-related field without problems in the color hue of the color image formed as well as in storability of the material is strongly desired.